This invention relates to a tool intended for chip removing machining of the type that comprises a shank having a clamping end and having a machining element at an opposite end.
During chip removing machining, such as turning or drilling, problems often arise with vibration, specially in cases where the length of the shank or the tool is at least 4-5 times larger than the diameter thereof. One type of vibration is bending vibration, the shank being bent to and fro and submitted to bending deformations. This phenomenon constitutes a common problem, for instance during turning, specially internal turning, where the shank in the form of a boring bar has to be long in order to reach the workpiece, at the same time as the diameter of the bar is limited by the dimension of the hole in which machining is carried out. During such drilling, turning and milling operations, where the distance to the workpiece is large, extension units are used, which frequently cause bending vibration leading not only to deteriorated dimensional accuracy and irregularities in the workpiece, but also to reduced service life of the milling cutter and its cutting insert or the machining element thereof.
Another type of vibration is torsional vibration, the shank being turned or screwed to and fro around the longitudinal axis thereof during which shear strain is created. Such vibration arises, for instance, during drilling, specially at higher speed. Also torsional vibration leads to a poorer quality of the machined surface, as well as reduced service life of tools and cutting inserts. An important nuisance is also the working environment problem which torsional vibrations give rise to, in that a shrill noise is generated during rotation.
Dampening of vibrations in tools for chip removing machining has previously taken place by pure mechanical dampening, the shank being formed with a cavity in which a mass of, for instance, counter-vibrating heavy metal is applied. In doing so, the weight and the position of the mass is tuned in order to bring about dampening of the vibration within a certain range of frequencies. The cavity is then filled with a viscous liquid, e.g. oil, and is plugged. However, this technique works tolerably well only in those cases where the length of the shank is approx. 4-10 times longer than the diameter thereof. In addition to this limitation, the pure mechanical dampening has an obvious disadvantage inasmuch as the range of frequencies within which the dampening acts is very limited. An additional nuisance consists of the weakening of the structural strength, which the cavity formed in the shank entails.
In entirely different technology areas, a development of more efficient, adaptive dampening techniques, has been started with the utilization of, among other things, piezo-elements. A piezo-element consists of a material, most often of a ceramic type, which on compression or strain in a certain directionxe2x80x94the direction of polarizationxe2x80x94generates an electric field in this direction. The piezo-element is usually in the shape of a rectangular plate with a direction of polarization, which is parallel to the major axis of the plate. By connecting the piezo-element to an electrical circuit, including a control module, and compressing or elongating the piezo-element in the direction of polarization, an electric current will be generated and flow in the circuit, electric resistive components included in the control module releasing heat according to known physics. In doing so, vibration energy is converted to thermal energy, whereby a passive dampening, but not totally neutralizing effect on the vibration is obtained. What is more, by forming the control module with a suitable combination of resistive and reactive components, so called shunts, selected frequencies may be brought to be dampened particularly effectively. Such frequencies are advantageously the so called xe2x80x9ceigenvaluexe2x80x9d frequencies of the exposed eigenmodes of the object in question, which are the ones that are especially excited.
Conversely, a piezo-element may be compressed or elongated by the fact that an electric voltage is applied over the piezo-element, and this may be used as a control device or operating device (actuator). This may, then be used for an active vibration reduction by the fact that the polarity of the applied electric voltage is chosen in such a way that the mechanical stress of the operating device acts in the opposite direction, as an external, mechanical stress, the emergence of vibration being suppressed by the fact that other kinetic energy, for instance energy of rotation, is not permitted to translate into vibration energy. The synchronization of the applied electric voltage in respect to the external mechanical tension, the effect of which should be counteracted, is then carried out by the fact that a feedback signal from a deformation sensitive sensor is fed to a control means in the form of a logical control circuit, e.g. a programmable microprocessor, in which the signal is processed to almost instantaneously control the electric voltage applied over the operating device. The control function, i.e. the relation between the input signal from the sensor and the output voltage, may then be made very complex. A self-learning system for adaptation to varying conditions is, for instance, feasible. The sensor may consist of a separate, deformation sensitive device, e.g. a second piezo-element, or be common with the operating device.
Examples of realized applications and current development areas for utilization of piezo-elements in vibration dampening purposes are described in Mechanical Engineering, November 1995, p. 76-81. Thus, skis for alpine skiing (K2 Four ski, K2 Corp., USA) have been equipped with piezo-element with the purpose of repressing undesired vibration, which otherwise decreases the contact with the ground and thereby reduces the skier""s prospect of a stable and controlled skiing. Furthermore, applications such as increased wing stability of aeroplanes, improved comfort in motor vehicles, suppression of vibrations in rotor blades and shafts of helicopters, vibration reduction of machining platforms for flexible manufacture, and increased hit precision of military weapons are mentioned. In data sheets from Active Control eXperts (ACX) Inc., USA (manufacturer of piezo-elements) vibration reduction of snowboards is also mentioned.
The present invention aims at managing the shortcomings of previously known tools for chip removing machining mentioned in the introduction and at providing a tool with an improved vibration dampening. Thus, a primary object of the invention is to provide a robust tool with the ability to efficiently dampening of vibrations over a wide range of frequencies, for instance where the length of the shank is 3-15 times larger than the diameter thereof, preferably approx. 4-6 times longer than the diameter thereof. It is also an object to provide a tool for chip removing machining having a longer service life for the tool itself as well as the cutting element thereof, compared to previously known tools. Additional objects of the tool are that the use thereof should lead to an increased quality of the surface of the machined workpiece and to an improved working environment by reduction of high frequency noise.
According to the invention, at least the primary object is attained by the features defined in the characterizing part of claim 1. Preferred embodiments of the invention are furthermore defined in the dependent claims.